The present invention relates to a method and apparatus for molding an article, such as a preform, which is used for making an information recorded disc, and particularly to such a method and apparatus which makes a preform which forms a disc having reduced voids and blisters in its surfaces.
Compression molded recorded discs are generally made by forming a preform of thermoplastic material and placing the preform centrally between a pair of heated molds. The molds are closed against the preform, melting the thermoplastic and compressing it, causing it to flow outwardly to fill the mold cavity and define the record contour. In molding the disc, it is highly desirable to prevent, or at least minimize, surface defects, such as voids and blisters caused by trapped air and gases, which can disrupt the surface relief pattern in the surface of the disc. The prevention of such surface defects is more desirable in making high density information discs, such as video discs, which utilize very high packing densities of the surface relief pattern, since even the smallest void or blister can disrupt a significant portion of the pattern.
An apparatus which will mold suitable preforms is described in U.S. Pat. No. 4,281,816 to Charles B. Carrol et al, entitled "Automatic Apparatus for Molding a Preform" issued Aug. 4, 1981, which is incorporated herein by reference. This apparatus includes a mold cup having opposed top and bottom mold members and a carrier arm between the mold members. The mold members have cooperating opposed recesses which, along with the carrier arm, mate to form a mold cavity. The bottom mold member has an opening therethrough through which the plastic mold material is admitted into the mold cavity. Pneumatic cylinders have piston rods which are connected to the mold members and the carrier arm for moving the mold members toward and away from each other and the carrier arm transversely across and away from the mold members.
In forming a preform with this apparatus, the mold members are moved together to a collapsed mold position in which the bottom mold member fits within the top mold member so that the mold cavity is of a volume smaller than the desired volume of the preform being molded. The plastic mold material is injected into the collapsed mold cavity to the bottom mold member until the mold cavity is filled. The injection of the plastic mold material is continued causing the top mold member to move away from the bottom mold member so as to expand the volume of the mold cavity until the mold cavity is of the desired volume. The injection of the plastic mold material into the mold cavity is then stopped. The mold members are then separated to permit removal of the molded preform by the carrier member.
In order to cause the movement of the top mold member so as to expand the volume of the mold cavity, the force applied by the continued injection of the plastic mold material must be greater than the force applied by the hydraulic cylinder attached to the top mold member. This force determines the density and shape of the preform which in turn determines whether the disc formed from the preform contains any surface voids and blisters. It has been found that with the apparatus shown in the Carroll et al patent the force applied to the preform builds up to a relatively high value, about 50 psi, and that the resulting preform causes an undesirable amount of blisters and voids in the surfaces of the disc made therefrom.